In power transmission equipment of all types there frequently is a need to provide a safety device which interrupts the transmission of torque, if the loading becomes too great. For example, an engine driving several pieces of equipment may, due to a mechanical failure and without the knowledge of the operator, have all the torque directed to only one of the pieces of equipment which is not designed to operate at such high torque. In such situations it is desirable to employ a torque limiter which, upon reaching a preselected level of torque, will interrupt the transmission of power and let the driving force run free or turn it off. The usual type of device used in the past for this purpose relied on shear pins or bolts which would break when a certain level of torque was reached, such as the machine described in U.S. Pat. No. 1,672,582 to Tenney. Shear pin devices are particularly vulnerable to failure by fatigue forces, which occur when the pin or bolt is subjected to repeating cycles of being stressed and then released. A shear pin device is, therefore, limited in its utility to a load at or below the fatigue endurance limit of the shear pin material. Thus, a shear pin can not be relied upon to function as a torque limiter at stresses between its fatigue endurance limit and its ultimate strength.
In order to avoid the disadvantages of a shear pin device there have been inventions employing spring loaded pawls, detents, and the like as a torque limiter that could be reset to function again after interrupting the transmission of power in its normal operation. U.S. Pat. Nos. 2,038,896 and 3,866,728 describe devices of this type. This type of torque limiter involves some radial movement before the power is interrupted, and because repeated use of the device results in wear of the radial movement components, there is a continuing change in the release torque over a period of time at which the device operates. This is an undesirable characteristic where one wants the device to function at a precise, unchanging torque level.
Friction clutch devices have also been used for torque limiters. Two opposed surfaces are pressed together with sufficient force to rotate together when one is driven and the other is not. If the frictional forces are sufficient, the power will be transmitted; and if the forces are insufficient, the surfaces will slip with respect to each other. These devices are unsuitable in many applications because there is no positive interruption of power transmission, but rather an irregular, chaotic series of slips and catches to provide a jerky transmission of some power.
U.S. Pat. No. 4,572,342 to Weiss et al. describes a torque limiter which combines the use of opposing faces of a frictional clutch member and spring biased ball devices as a clutch element. This device, however, suffers from the above-described problem of the maximum torque changing as wear occurs in the ball engaging surfaces permitting more and more radial movement before the power is interrupted.
It is an object of this invention to provide an improved torque limiter combining the best features of a shear pin device with a frictional clutch member. It is another object of this invention to provide a torque limiting device which has negligible radial movement, and therefore, a maximum torque transmission which does not change with time or with the wear of the device. It is still another object of the invention to provide a torque limiting device which is not subject to fatigue stresses that lower the maximum torque transmission by shear pins. Still other objects will appear from the more detailed description which follows.